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How To fasten rubberwood desktop to electric motorized steel base, in easy detach/ knock-down way using Fasteners? e.g. Wood inserts?

I'm designing & putting together a workstation desk with the following traits:

  • Electric motorized (lift capacity: 120 kg) & heavy Steel frame base (30kg)
    • Its a knock-down design put together using hex key-driven M6 M8 & few M10 bolts
  • Rubberwood desk/ tabletop

So for consistency with the above & need for easy disassembly, I'd prefer to use similar metric bolts.

Hence, I'm considering attaching the top with metric bolts from the bottom, through the pass through layers of a steel base and a rubber cushion spacer + washer (Total: ~11mm), into matching threaded inserts.

  • Upon reading online the insert types I found are these and am told that top left thread-in types for hardwood would be suitable.

  • Commenters/ responders here have posted there are special types of other better Fasteners. I'd like to know, review them for additional "holding power" and related benefits they bring.

4 types

The rubberwood top is 18mm thick & pass through non-wood layers (~11mm). I do not know the final dimensions of the top, but assume a rectangle roughly 1m wide and under 2m long. [The OP is free to update these dimensions as necessary --jdv]
(OP: Size subject to change but within above & hence need insert for detachability)

My main questions are about the fasteners/ inserts.

  • What is the safe maximum depth the inserts can be made into the rubberwood? Will this material hold inserts at all?
  • Which insert style is recommended in this application?
    • What special type of fasteners supersede wood inserts I've mentioned so far?

Follow-on question: Does the size of the bolt (M6, M8, M10, etc.) and matching insert matter for this application?

Update Follow-up to this Question:

Noticed a lot of variety of combinations even in threaded inserts of the hardwood type: Self tapping (advise by jdv), Slotted, 3 holes/ blind holes, collar, Hex, Reservoir, etc. (Pic from some German site)

(Image is big so linked, but can be inserted if SE folks ok - Or should these slot/ holes/ etc meta parameters go to a new Q?)

What pros cons of these in my scenario and what's recommended or to avoid? Why bother asking all this?

Scouted a dozen mom & pop hardware shops I went to did not have any.(Nothing like Home Depot).

I do see some of the above variety on local business yellow pages listing site, so they do exist & have to be found, but they don't respond to me/ consumers.

I'll eventually run into and find some variants, just not sure which ones - hence, I need to know the "yes, okay, maybe, avoid, no way" of some of the common types.

High Res measurement pics:

All the layers I want to sandwich together: Image 2

The metal and rubber layers: Image 1

Width of the rubber washer: Image 3

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    There is a lot of what feels like unnecessary detail here -- so much I can't quite discern the actual question. For example, I still don't know if you want through-holes through the material, or if you want the fasteners to be hidden (possibly from the bottom). An edit would help. Get rid of the crazy formatting and ask the specific question you want to ask. Don't forget to search here and DIY.SE for previous Q&A.
    – user5572
    Oct 20, 2021 at 12:18
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    So have you used the inserts? Or are you asking if you should? You have to account for wood movement. There are proper ways to go about attaching a tabletop.
    – gnicko
    Oct 20, 2021 at 16:04
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    This is very hard to understand but I think you start from a mistake. You should not use inserts for normal attachment. they allow bolts to be removed multiple times, is that what you seek?? If no, such inserts are not necessary. And also I don't think you allow for wood movement. All big tables require this!
    – Volfram K
    Oct 20, 2021 at 19:25
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    This question is poorly written and not of much use for anyone in the future. If not closed it should be drastically edited to remove all the unnecessary verbiage.
    – gnicko
    Oct 20, 2021 at 20:17
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    So table must be knockdown? ok ok I try to break it down for you. No need for more than M6 as these are already so strong. Wood screws to attach tops are usually thinner and hold is weaker! Brass or steel does not matter. Flat head or hex drive does not matter. You want screws min 5-6mm from surface, so 12 or 13mm inserts max. But 10mm no problem and more safe. For M6 coarse, every revolution = 1mm deeper. So bolts M6 x 20mm max. Minimum 4 threads engaged is strong enough, M6 x 16mm no problem if closest size available.
    – Volfram K
    Oct 21, 2021 at 23:21

3 Answers 3

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I would thread holes into tabletop because I know this is strong enough because wood screws are strong enough for same purpose, and there is no insert. But if you choose inserts here is my answer.

Allowance for wood movement to be addressed. We are not yet sure if this is needed or how much.

No need for more than M6 as these are already so strong. Wood screws to attach table tops are usually thinner and the hold is weaker!

Brass or steel does not matter. Both strong enough.

Flat head or hex drive does not matter, just 2 styles. No difference in strength. Hex easier to fit clean because driver size is exact.

You want screws min 5-6mm from surface, so 12 or 13mm inserts max. But 10mm no problem and more safe. You will not use all threads anyway. Be exact when you drill holes!

For metric coarse, pitch = advancement. Pitch of M6 is 1mm so every revolution = 1mm deeper. Gives M6 x 20mm max [11mm pass through metal frame and rubber rings + 9mm engagement in insert for 1mm clearance from bottom]

Minimum 4 threads engaged is strong enough, so M6 x 16mm no problem if closest size available.

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    Assuming a fair-sized rectangular top, we will definitely need to allow for wood movement. For this design the holes in the steel base should be made into slots along the direction of this movement across the grain. Otherwise, this is the most succinct and correct answer to the heavily edited question.
    – user5572
    Oct 22, 2021 at 15:08
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    @AlexS Wood movement: Changes in the relative humidity tend to cause wood to expand/shrink in a direction perpendicular to the wood grain. (the lines in the wood). A good summary can be found here: workshopcompanion.com/KnowHow/Design/Nature_of_Wood/… You may be most interested in the last section of the page.
    – gnicko
    Oct 22, 2021 at 18:59
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    @AlexS, I explained what wood movement is, and in what axis it works in my Answer just in case you (and any future readers) knew nothing about — I specified it was perpendicular to the grain, and put it more clearly and re-emphasised this by referring to width more than once. I don't know how to say it more plainly I'm sorry.
    – Graphus
    Oct 23, 2021 at 0:09
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    Because rubberwood I think has low movement then possibly there will be enough allowance for movement built into the desk from holes 12-13mm size. But this is 100% guess without real width. I will give you approximate values: for tabletop 600mm wide movement maybe is 11.7mm, perfect :) but if top is 1000mm wide 19.5mm :(
    – Volfram K
    Oct 23, 2021 at 6:53
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    @AlexS in your last picture, "Width of the rubber washer:", the grain (those lines you see in the surface) run in the same direction as the tape measure. There will be some but very minimal wood movement in that direction. So little that woodworkers do not worry about it. The movement will happen perpendicular to the grain (direction of the tape) and that is what you will have to account for. There will be cases where the grain is "circular" for small areas around knots, but that doesn't impact movement in any significant way.
    – FreeMan
    Oct 25, 2021 at 16:07
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Edit for the new information that this is a knockdown (K/D) project.

This probably does require the use of threaded inserts
Although you can thread wood directly1 inserts are intended to provide very durable threading for machine screws/bolts in wood and other softer materials, allowing for regular and repeated adjustments over time, especially under heavy load, or for complete removal and reinsertion of the fasteners as might be needed for knockdown furniture, especially if frequent disassembly is going to be needed.


You haven't asked about this but as mentioned already in the Comments it is an important factor.

Allowance for wood movement
The key thing with any tabletop made from solid wood is to allow for seasonal movement.

All solid wood experiences seasonal expansion and contraction perpendicular to the grain direction, see previous Answer to which direction does wood expand?

For any type of glued-up panel — regardless if made from many smaller pieces2 or two or more wider boards — or single very wide boards, where the width is approximately 460mm (~18") you need to start considering movement. At widths in excess of 600mm (24") some allowance usually becomes vital. The wider the tabletop the more allowance is needed.

From what we have been told up to to this point there may or may not be enough allowance naturally available in this desk due to the size of the holes in the metal frame. Note that the smaller the machine screws used here the more allowance is built it, because it leaves more space around each fastener — approximately 12-13mm for each pair of screws if M6 are used, based on the stated size of the holes.

See previous Answer for a little more on how you would normally attach a solid-wood tabletop.

If you don't build in some allowance for movement the table could experience serious problems. Normally the issues are breakage or tearing out of the screws, or warping or cracking of the tabletop. Here there's also a chance the steel framework could be distorted because the pressure exerted by expanding or shrinking wood can be considerable.


1 Threads in wood are of course more durable in some harder hardwoods, e.g. maple, but they can work even in softwoods and weak hardwoods like poplar. In softer or more crumbly woods (which includes some good hardwoods) the threading can be substantially reinforced by dribbling in some superglue into the hole.

2 These smaller pieces may be called staves, hence "stave construction" although regrettably this is now most commonly, and incorrectly, sold as "butcher block".

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  • Very interesting insights :) I totally get where you're coming from - but as you mentioned; My case - All this is because "Knockdown K/D" modularity/ ability is key for me. If I cannot knock it down with a Hex key, I am not buying it - I've slept on a floor with a yoga mat/ sleeping bag for years - even when I was near a home depot & ikea. My 1st buy an L shaped w scews & nails, was a god awful pain to deal with. 2nd was metal frame glass desk - hex knock down friendly - loved it. Just how life has been- Storage Units & nomadic. Maybe not for Most WW SE folks.
    – Alex S
    Oct 21, 2021 at 14:35
  • @VolframK - ^ More info on why.
    – Alex S
    Oct 21, 2021 at 14:41
  • Fair point on using woodscrews. My guess is that the OP is interested in construction that can be taken apart and put back together again. Heck, my router table is attached to a base using bolts and inserts just so I can remove it and put the scroll-saw (which hardly ever gets used, and really is just a free acquisition) in its place. I guess file that under "small shop problems".
    – user5572
    Oct 21, 2021 at 17:31
  • So the table IS to be knockdown? Why are we only hearing about this now? Didn't you think this would be relevant info to include in the Question? (As would that it's a standing desk BTW.) :-| Anyway put that aside for now, you haven't given the dimensions yet. Unless this desk is much smaller than I'm guessing you do have to account for movement, and attachment through a metal framework tends not to provide it (or at least enough of it). But it's impossible to advise on this front until you tell us what size it is.
    – Graphus
    Oct 22, 2021 at 8:10
  • @Graphus - I was just slammed above for "excessive" information :) - I figured using Inserts is an automatic indicator of hex key knock-down assemble/ disassemble. You all probably come from woodwork world, where im a newbie, as I come from the knock-down world. PS: Steel frame is 100% Hex drive M bolts - I did mention how I wanted to maintain that binding if i could.
    – Alex S
    Oct 22, 2021 at 17:40
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The question at hand is "how deep should I make a hole in relatively soft wood for a blind insert for an M-sized bolt?"

A good rule of thumb is don't remove more than 2/3rds the material in any given dimension when doing joinery. It isn't a hard and fast rule, but a nice start. In this case it'll work just fine because nothing here is critical.

The follow-on question, which size of M bolt to use is up to you. You want to choose the size appropriate for the entire dimension of the panel, with larger being better for shear and tensile strength. But your main challenge will be getting good thread engagement at such shallow depths, since it'll be the threads giving you that tensile strength. As long as you can safely move the item by gripping the panel this will probably work. But depending on the entire dimensions you may find that only 6 points of contact mean threads rip out easily no matter what.

For this wood use the screw-in self-tapping inserts, with or without glue.

Choosing more expensive fasteners made out of tougher material might be required.

Allowing for panel movement will be necessary in most cases. For this design that'll mean slotted holes in the metal along the axis where the panel is across the grain.

However, much of this is application driven. How strong does this have to be? If this is just a big table then typical installation and typical fasteners will be fine, even with only 1/3 fastener thread engagement. But maybe you need to be sure? In which case you can epoxy the inserts, use larger inserts designed to engage more fully with the material, and use fasteners with finer threads and higher strength (and, if possible, maximize your thread engagement).

A word on failure modes

There are two failure modes here:

  1. The inserts tear out of their self-tapped threaded holes. The weakest part of any metal-to-wood contact (or, indeed, any good glue joint) are the wood fibres around that contact. This is the most likely failure if the top is lifted too many times or if the frame is very heavy; the inserts will simply rip out. You could also tear them out simply by torquing the bolts down enough, as the leverage of threads is almost certainly stronger than the insert-to-wood joint. This can be somewhat mitigated by also using some sort of glue when installing the inserts.
  2. The metal threads in either the insert or the bolt fail, either because the material weakens, or is fatigued because of a thread-mismatch or cross-threading, or simple mechanical fatigue. Larger fasteners don't necessarily have larger threads, and most of the stress is going to be on the threads fully engaged in the insert. It is possible that these threads will let go from stress, or strip out over time if they are removed and replaced often. This is unlikely in normal use, but it is possible. There are a lot of cheaply made bolts out there that often have poor threads or use metal with poor strength. If this table was subject to a fair amount of vibration you can apply thread-locker to the bolt threads when installing them.
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  • Responses in order. Pls update within answer if possible- I'm told & read RW is a tropical hardwood, is it not? | So M10 > M8 > M6 ok | Clarify please "safely move the item by gripping panel" What item & what panel? | Rip out what? Inserts from wood or Bolts from Inserts? | So proper pilot hole with good insert thread cutting is key? | Fastner = bolt or insert? | Tougher material = ?? Brass/ SS ? | Panel =? slotted what in which metal? Outer Thread of Insert across Wood grain? | How do I get read up on the last para you wrote?
    – Alex S
    Oct 20, 2021 at 17:53
  • 2/3 of 18mm wood gives a 12mm insert/screw insertion depth. 1/2" (for those still using old measurements) seems like it should be more than enough to hold a table top to a frame. Try to avoid dropping large, heavy object (including drunken friends) on the overhanging edge of the top and don't lift the table by the top (maybe - no indication of how heavy the frame is), and all is likely to be good.
    – FreeMan
    Oct 20, 2021 at 17:54
  • @FreeMan I was told 12mm be enough so jdv scaring me :) maybe we both imagine diff things sorry jdv. Already info was so much. But To add clarify. Frame is heavy steel dual electric motorized 120kg lift capacity. Desktop will hold 2/3 monitors as before it did on ikea frame. No drunk jumping or lifting using wood top.
    – Alex S
    Oct 20, 2021 at 18:02
  • I'm not sure what you read in jdv's answer that's scaring you - he suggests removing no more than 2/3 the material. That makes a 12mm deep hole for the insert in your case. Since you've got a heavy frame with the added weight of motors to raise/lower it, just make sure to lift by the frame and not by the top when you're moving it and all should be good. If you need to move long distances & it's difficult to reach the frame with the top on, simply unscrew the top & move it in two pieces...
    – FreeMan
    Oct 20, 2021 at 18:14
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    @AlexS I don't know what those slots are for, but they don't seem to be for driving the insert in -- they are at the narrow end of the tapered insert which goes into the pilot hole. Perhaps the bolt is expected to expand the insert when installed to force the insert against the hole when in use? Sounds like a Q for DIY.SE.
    – user5572
    Oct 26, 2021 at 18:07

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